Estimating time in hours/days:
a. is done at the Story Point level so that appropriate time for tasks can be allocated
b. is done at the task level in Scrum to validate the number of Story Points that are planned.
c. is never done in Scrum.
d. is the primary estimation tool used in scrum for schedule and budget tracking.

Estimating time in hours/days:

a. is done at the Story Point level so that appropriate time for tasks can be allocated

Explanation:

By translating Story Points to hours you stop benefiting from the speed of relative estimation. You start working in hours and risk giving commitment, the concept is simple, yet difficult to apply and almost every Scrum team uses them but they are not part of the official Scrum Guide.

Story Points represent the effort required to put a PBI (Product Backlog Item) live, it represents a normal distribution of time. This is faster than estimating without any reference, when estimating in hours you make an accurate time commitment. Estimating in Story Points prevents giving an exact commitment but, nobody knows exactly how many hours you are appointing to an specific issue.

a) carbon, hydrogen

b) carbon, hydrogen, oxygen, nitrogen

c) carbon, hydrogen, oxygen, nitrogen, phosphorus

d) sugar

e) amino acid

f) nucleotide